1. Field of the Invention
Our invention relates to the field of encrypted data communication, and particularly to remote keying by means of a commonly held, long term, nonextractable master variable.
2. Background of the Invention
Encrypted communications equipment has traditionally been used primarily for military and diplomatic communications. A major cause of this limited use was the relatively high costs involved in insuring the security of the equipment and the enciphering keys. Elaborate procedures were required to produce, transport, and store the keying materials prior to use, and also to insure the integrity of the personnel to whom the materials were entrusted. It also came about that a great diversity of equipments were designed and built, each capable of communicating information only to another equipment of identical design.
Modern electronic communications systems including enciphering capabilities are coming to be used with increasing frequency by non-governmental organizations for communication of important business information. There is an obvious need to protect information such as funds transfer requests, company proprietary information and business decisions from interception by unauthorized persons. Federal, state and local governments are also finding a need for encrypted communications for other than military purposes. As more and more organizations begin to need and use data encryption capability, it becomes increasingly desirable that such equipments be interoperable with the greatest possible number of other equipments. For the mutual benefit of all potential purchasers, it is further desirable that such equipment be made available at the lowest possible cost.
In order to insure a standard of protection for U.S. government non-military information, and in order to promote the availability of high-quality data communications protection to the public at a reasonable cost, the U.S. National Bureau of Standards has adopted a standardized data encryption algorithm. The algorithm has been published as Federal Standard 1026, which is available to the public through the Government Printing Office in Washington, D.C. as Federal Information in Processing Standard 46. One purpose of the Government in creating the Data Encryption Standard (DES) is to provide a highly secure and interoperable class of encryption equipment which may be produced by the private sector of the economy in numbers large enough to reduce costs and stimulate purchase by users who need such a capability.
A DES-based device has three modes of operation; electronic codebook, cipher-block-chaining, and cipher feedback. In the cipher feedback mode, the DES is a pseudorandom number generator whose output is used to encipher from 1 to 64 bits of plaintext for transmission. A frequently used embodiment of DES will produce an output of 64 key bits per cycle, 56 of which will not be used for encryption. The remaining eight bits will be modulo-2 added to eight bits of plaintext to create 8 bits of encrypted data. The encrypted data is transmitted and also fed back into the DES input to replace the eight bits oldest in time in the register. When two such devices are used to provide cryptographic protection to a communications link, at each end of the link the operator loads a common keying variable by whatever means provided (thumbwheel, keyboard, paper tape, etc.). At the commencement of transmission, the transmitting operator causes an initializing vector to be sent to the receiving unit. This initializing vector is also entered into the DES input of the transmitter unit as the starting information. This initializing vector serves two purposes: it synchronizes the two cryptographic devices and it assures that each transmission will commence at a new and unique point in the key cycle. Thus the published encryption process includes manual loading of the variable and the modification of that variable to some different, unique starting point.
It is readily apparent from the above that many traditional disadvantages associated with enciphering equipment are still present, such as the continuing need to produce, transport, and store the required common keying variables. Because the secrecy of the communication is dependent solely on the continued secrecy of the key variable, it remains essential that employees to whom the variable is entrusted be trustworthy. Elaborate measures must be taken to prevent theft of the variable at all stages of its existence. The security clearance and physical protection measures employed by the military are obviously beyond the financial and practical capabilities of most commercial concerns.
It is possible to eliminate some of the transportation and storage of keying variables by the incorporation of remote keying. U.S. Pat. No. 4,182,933 to Rosenblum describes a secure communication system in which keying materials are themselves transmitted in enciphered form to terminals between which it is desired to communicate private information. A major complexity of this apparatus comes from the need for a separate key distribution center for the purpose of establishing an initial connection between the two terminals. While the Rosenblum apparatus provides a high degree of security, it retains the deficiency of requiring encryption of data between the subscriber and the key distribution center by means of a key variable which was commonly held by each prior to the initiation of the communication.